Conductivity enhancement of thin transparent films based on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) by a solution-processed route involving mixture of an organic acid and organic solvent is reported. The combined effect of p-toluenesulfonic acid and dimethyl sulfoxide on spin-coated films of PEDOT-PSS on glass substrates, prepared from its commercially available aqueous dispersion, was found to increase the conductivity of the PEDOT-PSS film to ∼3500 S·cm(-1) with a high transparency of at least 94%. Apart from conductivity and transparency measurements, the films were characterized by Raman, infrared, and X-ray photoelectron spectroscopy along with atomic force microscopy and secondary ion mass spectrometry. Combined results showed that the conductivity enhancement was due to doping, rearrangement of PEDOT particles owing to phase separation, and removal of PSS matrix throughout the depth of the film. The temperature dependence of the resistance for the treated films was found to be in accordance with one-dimensional variable range hopping, showing that treatment is effective in reducing energy barrier for interchain and interdomain charge hopping. Moreover, the treatment was found to be compatible with flexible poly(ethylene terephthalate) (PET) substrates as well. Apart from being potential candidates to replace inorganic transparent conducting oxide materials, the films exhibited stand-alone catalytic activity toward I(-)/I3(-) redox couple as well and successfully replaced platinum and fluorinated tin oxide as counter electrode in dye-sensitized solar cells.

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Self-organized semiconductor-semiconductor heterostructures (3R-2H) that coexist in atomically thin 2D monolayers forming homojunctions are of great importance for next-generation nanoelectronics and optoelectronics applications. Herein, we investigated the defect controlled growth of heterogeneous electronic structure within a single domain of monolayer WS to enable in-plane homojunctions consisting of alternate 2H semiconducting and 3R semiconducting phases of WS. X-ray photoelectron, Raman, and photoluminescence spectroscopy along with fluorescence and Kelvin probe force microscopy imaging confirm the formation of homojunctions, enabling a direct correlation between chemical heterogeneity and electronic heterostructure in the atomically thin WS monolayer. Quantitative analysis of phase fractions shows 59% stable 2H phase and 41% metastable 3R phase estimated over WS flakes of different sizes. Time-resolved fluorescence lifetime imaging confirms distinct contrast between 2H and 3R phases with two distinct lifetimes of 3.2 ns and 1.1 ns, respectively. Kelvin probe force microscopy imaging revealed an abrupt change in the contact potential difference with a depletion width of ∼2.5 μm, capturing a difference in work function of ∼40 meV across the homojunction. Further, the thermal stability of coexisting phases and their temperature dependent optical behavior show a distinct difference among 2H and 3R phases. The investigated aspects of the controlled in plane growth of coexisting phases with seamless homojunctions, their properties, and their thermal stability will enable the development of nanoscale devices that are free from issues of lattice mismatch and grain boundaries.

The realization of next-generation transition-metal dichalcogenide based nanoscale devices demands stringent control over coherent in-plane heterostructures of atomically thin monolayers with exceptional properties. In this paper, we report atmospheric-pressure chemical vapor deposition growth of largedomain, coherent polymorphic in-plane heterostructures of monolayer WS 2 on a SiO 2 /Si substrate with intriguing optical and electronic properties. The formation of in-plane heterostructures with 1H and 1T polymorphs was extensively analyzed using a variety of spectroscopic as well as microscopic techniques, along with lifetime luminescence imaging. High-angle annular dark-field scanning transmission electron microscopy revealed coexistence of the heterophases in monolayer WS 2 heterostructures. Back-gated photoconductivity measurements in nanoscale field-effect-transistor device geometry and the rational design of a WS 2 heterostructure pattern demonstrate optoelectronic applications.